1. Field of the Invention
The present invention relates to an improvement of a super-twisted nematic liquid crystal display. In particular, the present invention relates to a STN type color liquid crystal display device to effect multicolor development without using a color filter.
2. Discussion of Background
When a passive addressing type liquid crystal display device such as a twisted nematic liquid crystal display device (TN-LCD) or a super-twisted nematic liquid crystal display device (STN-LCD) is driven for effecting a display, driving voltages are applied between electrodes according to a line successive selection method wherein either of a row electrode or a column electrode is determined as a scanning electrode and the other is determined as a data electrode.
As a typical line successive selection method, APT (Alt-Pleshko Technique) or IAPT (Improved APT) has been known. This technique is desirable for a multiplex driving method because ON-OFF levels can easily be produced.
Further, in the line successive selection method, scanning signals are successively supplied to scanning electrodes; data signals are supplied to data electrodes, and potential differences between the scanning signals and the data signals are applied to pixels. In this case, some driving methods are employable so as not to cause a change of the effective voltage applied to a certain pixel due to another pixel which undergoes an ON display or an OFF display. One of the driving methods is called a voltage averaging method which is the basic method for driving an effective value responsive type liquid crystal display device.
Description will be made on the driving method for a liquid crystal display device with reference to FIG. 3. FIG. 3 shows a basic waveform used for driving a passive addressing type liquid crystal display device, which is an example of the line successive selection method using the voltage averaging method wherein a scanning signal voltage is expressed by V.sub.r, a data signal voltage is expressed by V.sub.c and the number of scanning electrodes is expressed by N. In the sequence for driving pixel signals, there are a selection type and a non-selection time wherein a voltage applied to a pixel in the selection time is (V.sub.r +V.sub.c) at the time of an ON display and the voltage is (V.sub.r -V.sub.c) at the time of an OFF display.
In the liquid crystal display device, there appears a change of orientation of liquid crystal molecule depending on a voltage to be applied. Since the change of orientation causes an optical change, the voltage difference between an effective voltage in an ON display and an effective voltage in an OFF display should be large as possible. When the scanning signal voltage is expressed by V.sub.r, the data signal voltage is expressed by V.sub.c and the number of scanning electrodes is expressed by N, the effective voltage in an ON display and the effective voltage in an OFF display can be expressed by the following Formulas 1A and 1B. EQU Effective voltage in an ON display=((V.sub.r +V.sub.c).sup.2 +(N-1).times.V.sub.c.sup.2)/N).sup.0.5 Formula 1A EQU Effective voltage in an OFF display=((V.sub.r -V.sub.c).sup.2 +(N-1).times.V.sub.c.sup.2)/N).sup.0.5 Formula 1B
The ratio of an effective voltage in an ON display part to an effective voltage in an OFF display part is called an ON-OFF ratio. Conventionally, when the passive addressing type liquid crystal display device is driven by the voltage averaging method, the ratio of the scanning signal voltage to the data signal voltage has been determined so as to increase the ON-OFF ratio. EQU ON-OFF ratio=((a.sup.2 +2a+N)/(a.sup.2 -2a+N)).sup.0.5 Formula 2
(where a=V.sub.r /V.sub.c, hereinbelow, (a+1) is called a bias value B.sub.R)
Accordingly, in order to maximize the ON-OFF ratio, the scanning signal voltage and the data signal voltage should be determined as shown in the following formula 3B. The bias value B.sub.R which can maximize the ON-OFF ratio is called the optimum bias value B.sub.RMAX. Actually, the applied waveform (IAPT) as shown in FIG. 2 is used to lower the driving voltage. EQU a+132 B.sub.R Formula 3A EQU B.sub.RMAX =P.sub.N Formula 3B
If a liquid crystal display device for a monochrome display is used and the liquid crystal is completely responsive to the effective value, the contrast ratio can be increased by increasing the ON-OFF ratio. However, when a fast responsive type liquid crystal is used wherein the sum of response time at a rising time and a falling time is about 200 ms or less, there is a change of orientation in a short time and accordingly, the contrast ratio decreases. Such phenomenon is called a frame response.
The frame response phenomenon appears remarkably as the voltage difference of pixel signals between the selection time and the non-selection time is larger. Accordingly, it has been known to control the frame response by lowering a bias value from the optimum bias value. However, the lowering of the bias value from the optimum bias value results in the reduction of the ON-OFF ratio, which reduces the contrast ratio.
Further, it has been known to increase the frame frequency in order to control the frame response. However, this technique had a drawback to increase the power consumption rate. Further, it has been known to use a multiple line addressing method (MLA method) to control the frame response. However, this method requires driving ICs for exclusive use.
Further, there has been known a color liquid crystal display device utilizing the mutual interference of the birefringence in each of the liquid crystal layer and the birefringent plate. For example, Japanese Unexamined Patent Publication JP-A-8-292434 proposes a super-reflective color liquid crystal display device (hereinbelow, referred to as SRC-LCD including a transmission mode) in detail, which provides an achromatic (white) display in a state of the driving voltage being OFF; facilitates use of multiplex driving, and effects a multicolor display.
In order to provide a color display of three colors or more by the SRC-LCD, it is necessary to apply an intermediate voltage. In this case, it has been known that the color purity in employing the multiplex driving is decreased in comparison with a change of color to a change of voltage in a case of using a static driving method. This is a phenomenon different from the case caused by the reduction of the contrast ratio in a conventional monochrome display.
Further, the sum of response time at the rising and falling in a SRC mode was about 400 ms, and it was thought that the influence of the frame response due to a fast responsive property of the liquid crystal, which was often seen in the conventional monochrome display type STN-LCD, was small.
In order to drive a STN-LCD, the optimum bias is generally used because the highest ON-OFF ratio can be obtained. Generally, a monochrome STN-LCD is driven under such condition. The optimum bias is defined as (N.sup.0.5 +1). For instance, the optimum bias value in using 64 lines is 9. SRC-LCD (portable calculator) operable by such driving method has already been known. Further, there has been known SRC-LCD with 240 lines which provides a VGA display under the driving condition of a bias ratio of 16 (the optimum bias ratio=16.5).
In the following, a conventional technique in which the bias ratio is changed will be described.
Digest of technical papers for SID 1991 describes in P.747-P.750 that when a fast response type STN liquid crystal device is used for multiplex driving, the frame response is large and the bias value should be smaller than the optimum bias in order to increase the contrast ratio. Specifically, when a liquid crystal device having a liquid crystal layer gap of 5 .mu.m, .DELTA.nd of 0.66 .mu.m and a response time of 75 ms and when it is driven under conditions of a frame frequency of 70 Hz and a duty ratio of 1/200, the optimum bias ratio shows 1/15.14. However, the SID1991 describes that use of the optimum bias of about 1/10 is better in order to increase the contrast ratio. Thus, the prior art document proposes to reduce the bias value in order to prevent the reduction of the contrast ratio and so on in a case of using the fast response type liquid crystal.
On the other hand, the present invention is to prevent the color purity from deteriorating due to a strong influence by the frame response because .DELTA.nd of the liquid crystal layer is large. For this, the bias value is determined to be smaller. Such tendency is obtainable even when the response time of liquid crystal is long (The average response time (.tau.ON=.tau.OFF) of the liquid crystal used for the present invention is about 200 ms).